1. Field of the Invention
The present invention relates to an optical head of a disk recording and reproducing apparatus in which an optical spot is projected onto a disk-shaped information recording medium, thereby recording and reproducing information optically.
2. Description of the Prior Art
Recently, an optical head and a disk recording and reproducing apparatus have been used for various applications, for example, DVD, MD, CD, CD-R, etc., an apparatus having a high density, high performance, high quality, and high added value have been demanded. In particular, in the magneto-optical disk recording and reproducing device using a magneto-optical media capable of recording, demands for portable type apparatus tend to greatly increase. Consequently apparatus having a small size, thin shape, high performance and low cost has been increasingly demanded.
Hitherto, a great deal of investigation concerning techniques for an optical head of a disk recording and reproducing apparatus for magneto-optical disk has been reported. The following is an explanation for one example of a conventional optical head of a disk recording and reproducing apparatus for magneto-optical disk with reference to the drawings. FIGS. 14, 15, 16, 17 and 18 are views to illustrate a schematic configuration and the operation principle thereof.
In FIGS. 14, 15, 16, 17 and 18, reference numeral 101 denotes a silicon substrate. 102 denotes a semiconductor laser light source fixed on the silicon substrate 101. 103 denotes a multifractionated photodetector formed on the silicon substrate 101 by an IC process. 104 denotes a radiator plate for holding the silicon substrate 101 in a state of transferring heat. 105 denotes a terminal wired from the multifractionated photodetector via a wire bonding, etc. 106 denotes a resin package holding the silicon substrate 101, the radiator plate 104 and the terminal 105. 107 denotes a hologram element (diffraction grating) formed of resin. 108 denotes a composite element including a beam splitter 108a, a mirror 108b and a polarization dividing element 108c. 
Furthermore, in this configuration, the silicon substrate 101, the semiconductor laser 102, the multifractionated photodetector 103, the radiator plate 104, the terminal 105, the resin package 106, the hologram element 107 and the composite element 108 are defined as an integrated unit 109. Reference numeral 110 denotes a reflection mirror. 111 denotes an objective lens. 112 denotes an objective lens holder to which the objective lens 111 is fixed. 113 denotes a magneto-optical recording medium that is an information recording medium having a magneto-optical effect. 114 denotes an objective lens driving device for driving the objective lens 111 in the focus direction (the direction substantially vertical to the magneto-optical recording medium 113) and in the radial direction (the direction substantially parallel to the magneto-optical recording medium 113) of the magneto-optical recording medium 113.
The objective lens driving device 114 includes components such as the objective lens 111 forming an optical spot on the magneto-optical disk by using a light flux released from the semiconductor laser 102, the objective lens holder 112, a base 115, a suspension 116, a magneto-optical circuit 117 and coils 118a and 118b. By electrifying the coil 118a, the objective lens 111 can be driven in the focus direction, and by electrifying the coil 118b, the objective lens 111 can be driven in the radial direction. Reference numeral 119 denotes an optical bench, which fixes the reflection mirror 110.
Furthermore, the integrated unit 109 is fixed by bonding the optical bench 119 to the resin package 106. As a result, the position of the optical bench 119 is determined so that the multifractionated photodetector 103 is positioned in the Z-axis direction (direction of the optical axis) in the position in which the region 124 of receiving a focus error signal is positioned substantially in the middle between the focuses 130 and 131 of the optical spot.
On the other hand, in FIG. 18, reference numeral 120 denotes an optical spot for detecting a focus error signal, formed on the multifractionated photodetector 103. 121 denotes an optical spot for detecting a tracking error signal formed on the multifractionated photodetector 103. 122 denotes a main beam (P polarization) formed on the multifractionated photodetector 103. 123 denotes a main beam (S polarization) formed on the multifractionated photodetector 103. 124 denotes a region of receiving a focus error signal. 125 and 126 are regions for receiving a tracking error signal, 127 denotes a region for receiving an information signal, 128 denotes a subtracter and 129 denotes an adder.
Furthermore, in FIG. 17, reference numerals 130 and 131 respectively are focuses of the optical spot for detecting a focus error signal and 132 denotes an optical spot formed on the magneto-optical recording medium 113.
In FIGS. 16A and 16B, reference numeral 133 denotes a cover, 134 denotes an adhesive and 135 denotes a flexible circuit.
Furthermore, as shown in FIG. 14, an optical head feeding device for moving the optical head in the radial direction of the magneto-optical recording medium 113 includes a feeding screw 136, a jackshaft 137, a feeding motor 138, a gear 139a, a gear 139b, a nut plate 140 formed on the cover 133, a bearing 141, and the like and is attached to a mecha base (a base on which a mechanism is disposed) 142 (details are not shown in the drawings). At this time, the nut plate 140 and the feeding screw 136 are fitted with each other, and the entire optical head moves in the radial direction due to the rotation of the feeding motor 138 by a feeding amount determined by the reduction ratio. The reduction ratio is determined by the gear ratio of a gear 139a to a gear 139b and by the pitch of the feeding screw 136.
Furthermore, at this time, the relative positions of the objective lens 111 and the optical bench 119 are displaced by the feeding amount. Furthermore, the maximum value of the radial direction moving amount of the objective lens 111 is a value right before the feeding motor 138 is rotated.
As shown in FIGS. 14, 15 and 20A–20C, the operation of the objective lens 111 at the time of recording or reproducing information on the inner periphery to the outer periphery of the magneto-optical recording medium 113 is explained. First of all, the objective lens 111 is positioned in the vicinity of the design optical axis. Then, electric current is applied to the coil 118b in order to move the objective lens 111 in the radial direction so that the objective lens 111 follows the track of the magneto-optical recording medium 113. Then, the voltage corresponding to the value of electric current applied to the coil 118b is applied to the feeding motor 138, and the feeding motor 138 is rotated when the voltage reaches a predetermined value, and thereby the feeding amount corresponding to the gear ratio determined by the gears 139a, 139b and the feeding screw 136 is applied to the optical head so as to drive the entire optical bench 119 in the outer peripheral direction. At this time, the relative displacement between the objective lens 111 and the optical bench 119 (or design optical axis) is a value obtained by subtracting the feeding amount of the optical head from the moving amount of the objective lens 111.
The following is an explanation of the operation of the conventional optical apparatus configured as mentioned above with reference to FIGS. 14, 15, 16, 17 and 18.
Light beams emitted from the semiconductor laser 102 are split into a plurality of different light fluxes by the hologram element 107. The plurality of different light fluxes pass through the beam splitter 108a of the composite element 108, are reflected by the reflection mirror 110 and converged into an optical spot 132 having a diameter of about 1 μm on the magneto-optical recording medium 113 by the objective lens 111 fixed to the objective lens holder 112.
Furthermore, a light flux reflected by the beam splitter 108a of the composite element 108 is incident in the laser monitor photo-receiving element (not shown) to control the driving current of the semiconductor laser 102.
The light beam reflected by the magneto-optical recording medium 113 travels in the opposite direction, is reflected and split by the beam splitter 108a of the composite element 108 and then is incident in the mirror 108b and a polarization splitting element 108c of the composite element 108.
The semiconductor laser 102 is provided in such a way that the polarization direction is parallel to the paper of the FIG. 17A. The incident light beams are split into two light fluxes having polarization components perpendicular to each other, and then are incident in the region 127.
Furthermore, the light fluxes that pass thorough the beam splitter 108a among the light beams reflected by the magneto-optical recording medium 113 are split into a plurality of light fluxes by a hologram element 107 and converged into the region 124 and the regions 125 and 126. The focus servo is carried out by a so-called SSD method and the tracking servo is carried out by a so-called push-pull method.
Furthermore, by calculating the difference between the main beam 122 including P polarization and the main beam 123 including S polarization, the magneto-optical disk information signal can be detected by the differential detection method. Furthermore, by calculating the sum of them, a pre-pit signal can be detected.
In the optical head configured as mentioned above, in order to obtain the desired detection signal by the use of the reflected light from the magneto-optical recording medium 113, the relative positions of the semiconductor laser 102, the objective lens 111 and the multifractionated photodetector 103 are adjusted during assembly. With reference to the adjustment of the relative positions, the setting of the initial location of the focus error signal is determined by specifying the dimension of the optical bench 119 and the resin package 106 of the integrated unit 109 so that the region 124 is located substantially in the middle of the focuses 130 and 131 of the optical spot.
Furthermore, as shown in FIGS. 16A and 16B, the adjustment of the tracking error signal is carried out so that outputs of the regions 125 and 126 are substantially uniform by maintaining the base 115 by the use of an external jig (not shown) and by moving the objective lens driving device 114 in the Y direction and the X direction.
This adjustment results in aligning the center of the axis of light emitted from the semiconductor laser 102 with respect to the center of the objective lens 111 in FIG. 17. Furthermore, the adjustment of the relative tilt of the magneto-optical recording medium 113 and the objective lens 111 is carried out by maintaining the base 115 by the use of an external jig (not shown) and carrying out the skew adjustment θR in the radial direction (around the Y axis) and the skew adjustment θT in the tangential direction (around the X axis). After the adjustment, the base 115 is fixed to the optical bench 119 by using adhesive 134. As mentioned above, the adjustment of the focus error signal and the tracking error signal and the skew adjustments are completed and thus the optical head is completed.
On the other hand, FIGS. 19A–19B show the focus servo of the optical head having a conventional configuration as mentioned above. In this configuration, the off-set amount to the GND with respect to the focus error signal calculated by the so-called SSD method and electric current corresponding to the off-set amount is applied to the coil 118b, and thus the focus servo is converged around the GND. The focus error signal generates a so-called S-letter signal due to the change of the position in the focus direction of the objective lens 111, and the focus point of the objective lens 111 is converged around the GND of the focus error signal. At this time, the defocus amount is defined by the difference between substantially the center of the S-letter signal and the GND as shown in FIG. 19B.
However, the optical system of the optical head having the above-mentioned conventional configuration is a so-called finite system. In this system, as the objective lens 111 moves in the radial direction of the magneto-optical recording medium 113 (in particular, with increasing distance from the design optical axis of the objective lens 111), the off-axis aberration on the optical spot 132 on the magneto-optical recording medium 113 and the change in the shape of the optical spot 120 for detecting a focus error signal on the region 124 are generated. Thus, displacement of the focus point of the optical spot 132 with respect to the magneto-optical recording medium 113 occurs, and thus defocus occurs as shown in FIG. 19B and FIG. 20C.
As shown in FIG. 21, when the objective lens 111 moves in the radial direction, off-axis aberration (wave aberration) includes astigmatism, coma-aberration, spherical aberration, high-order aberration, and the like. The most significant is astigmatism, and the defocus amount generated when the objective lens 111 moves in the radial direction increases as the radial direction moving amount of the objective lens 111 is large, and as the thickness of the objective lens 111 reduces. In particular, the optical head for a portable type disk recording and reproducing apparatus is required to be smaller and thinner, and as the objective lens 111 becomes smaller and thinner, the off-axis aberration increases.
Furthermore, when defocus occurs, the spot diameter of the optical spot 132 on the magneto-optical recording medium 113 becomes larger and the ellipticity of the spot increases. At this time, on the design optical axis (an optical axis having the radial direction shift amount of the objective lens 111 of 0 or a design optical axis of the objective lens 111), the direction of astigmatism at the optical spot 132 after released from the objective lens 111 by astigmatic difference of the semiconductor laser 102 is a direction in which the back-side line focus of the optical spot 132 substantially coincides with the radial direction. At the same time, as the objective lens 111 moves in the radial direction, the off-axis aberration (mainly astigmatism) increases. In particular, when the objective lens is thinned, in order to make the optical head small and thin, the off-axis (astigmatism) of the objective lens 111 increases.
Therefore, due to the defocus accompanied with the radial direction movement of the objective lens 111, the shape of the optical spot 132 on the magneto-optical recording medium 113 becomes an oval shape having a longitudinal axis in substantially the radial direction. When a groove neighboring a groove to be reproduced is irradiated with a part of the optical spot 132, there is a problem in that the recording and reproducing performance is deteriorated because of the reduction of the signal reading performance due to the increase of cross-talk when reproducing the information signal recorded on the magneto-optical recording medium 113 or because of the reduction of the reading performance by cross-talk of a wobble signal having address information, etc. (ADIP signal or ATIP signal) formed on the magneto-optical recording medium 113.
Furthermore, as shown in FIG. 21, in the small and thin lens, the off-axis aberration radically tends to increase. Therefore, with the increase of astigmatism, coma-aberration, spherical aberration or high-order aberration in accordance with the increase of the off-axis aberration and by the tilt of the objective lens, the optical performance is deteriorated radically with respect to the design axis. Thus, there is a problem in that the recording and reproducing performance is deteriorated radically. In FIG. 21, the degrees of the angle of view correspond to the moving amount of the objective lens 11.
With the foregoing in mind, it is an object of the present invention to provide an optical head capable of realizing a stable recording and reproducing with less cross-talk and capable of making the device to be small and thin by making the objective lens to be small and thin, and a disk recording and reproducing apparatus using the optical head.